1. Field of the Invention
This invention relates generally to systems, and their methods of use, that distribute laser pulses over surfaces of optical elements, and more particularly to systems, and their methods of use that distribute laser pulses over surfaces of optical elements to substantially enhancing lifetimes available from the optical elements.
2. Description of the Related Art
Solid state lasers high brightness beams are known to be an excellent tool for precision processing of materials and are increasingly in demand for a host of applications ranging from microlithography to marking and via drilling. For many semiconductor microprocessing applications, diode pumped solid state lasers, and their harmonics, offer a technology of choice because of their superior reliability, low operating costs and excellent output characteristics. In particular, a combination of high peak power, excellent mode quality and high long-term pointing stability enabled the development of reliable, frequency tripled and quadrupled high repetition rate lasers with ever increasing UV outputs near 355 nm and 266 nm, respectively.
Alternative solid state gain media, such as Nd:YVO4, Nd:YAG and Nd:YLF, have been utilized to generate high power UV outputs using frequency multiplexing with various nonlinear crystals. Among these, Nd:YVO4 has become a gain material of choice for applications requiring operation at very high repetition rates ranging, typically, from 10 to over 100 kHz. Frequency tripled lasers based on this medium are now commercially available with UV output powers of over 4 W at 355 nm. Fourth harmonic power levels exceeding 2 W at 266 nm have also been demonstrated in practical systems and interest has been growing in lasers with still shorter wavelengths such as the frequency-quintupled radiation near 213 nm and even shorter. UV lasers based on other gain materials, such as Nd:YAG, have been successfully power scaled as well, generally for applications requiring lower repetition rates but higher energies and/or longer pulse durations.
The increasing emphasis on power scaling at shorter wavelengths and higher repetition rates place difficult requirements on the laser components. A major practical limitation to continued scaling of power is the deterioration in lifetime of key optical elements, both linear and nonlinear. In particular, laser induced damage is known to compromise long term operation of protective coatings, substrates and the nonlinear materials employed in frequency conversion processes when subjected to high peak and average power laser beams. The literature recounts various mechanisms that can lead to such damage, including thermal, photoacoustic and plasma effects. The damage is known to accelerate the higher the power density and the shorter the wavelengths, and is further facilitated by the presence of defects on optical elements, which can form absorbing centers.
To date, development of damage resistant high quality coatings suitable for high power operation in the UV lags well behind coatings available at visible wavelengths. As repetition rate is increased, single pulse damage is further aggravated by the potential for cumulative damage mechanisms. Thus, allowing a high power beam to pass through a single spot in a coated optical element or a nonlinear crystal for long periods of time is known to result in performance degradation, sometimes at power levels well below single pulse damage thresholds. Mechanisms suggested for such cumulative damage include formation of UV absorbing color centers and structural changes of the polished entrance/exit faces of the coated element. Generally, such degradations become more severe the higher are the incident beam power densities and repetition rates and the shorter are the input and/or output wavelengths.
Nonlinear crystals employed in frequency converted high repetition rate laser system are especially susceptible to such cumulative damage, as manifested by the early onset of degradation in the harmonic conversion efficiency. Furthermore, in homogeneities present in any birefringment crystal can result in widely varying conversion efficiencies in different parts of the same crystal, a problem that is exacerbated when high intensity focused beams and temperature tuning are used to optimize the harmonic generation process. Nonuniformities in temperature throughout the crystal volume caused by varying distance from the thermal source or sink, contaminants, varying degrees of surface polish, and bulk irregularities can all compromise the crystal performance over time. Even before the onset of actual damage, thermal effects caused by residual UV absorption can lead to thermal dephasing which reduces the effective interaction length in the crystal and lowers the efficiency for frequency conversion.
Temperature and angle tuned non-linear borate crystals, such as LBO and BBO and the newly developed CLBO that are routinely used to produce frequency conversion at the third and fourth harmonics, are known to be subject to such thermal dephasing at high average powers. As repetition rates and pulse energies are increased, thermal dephasing can become an issue even for a material such as CLBO which has a large thermal acceptance bandwidth. Although this effect may be temporary and can further be alleviated using active temperature controls, the implementation of such techniques becomes more complex and costly as powers are increased beyond certain levels.
In particular, because of the generally low thermal conductivity of an isotropic crystals, the time constant for crystal temperature adjustment is too large to rely on temperature adjustment as the sole means for maintaining constant levels of UV output at elevated power levels. Similarly, though some of the observed damage mechanisms in crystals and other optical elements may be annealed over time, strong thermal effects due to increasingly high absorption will eventually result in permanent damage, requiring replacement of the element.
As is the case for other coated optical elements, the damage to nonlinear optical elements is more pronounced, and threshold for damage lower, as the output wavelengths become shorter. This has been a major limiting factor on achieving efficient conversion to higher order harmonics at scaled power levels.
In recent years, considerable efforts were carried out to mitigate against laser induced damage including improvements in the quality of optical substrates, surfaces and coatings, as well as the development of new, more tolerant laser and nonlinear conversion designs. One particular approach commonly employed in commercial systems containing harmonic modules is to translate the nonlinear crystal through the beam during operation so that the incident beam continually encounters a fresh crystal volume before any crystal degradation can occur.
For example, U.S. Pat. No. 5,179,562 to Marason et al teaches a system and means for crystal translation applied to the case of intracavity conversion of CW beams. This patent further describes method for active adjustment of the intracavity intensity and temperature profiles of the crystal to maintain optimal conversion efficiency levels. In another example, more specifically adapted to pulsed operation of solid state UV lasers, U.S. Pat. No. 5,825,562 to Lai et al., discloses a system providing continuous motion for minimizing laser exposure time for any one spot and prolonging the usable life of an optical element subjected to high intensity irradiation. The preferred embodiment of the system of Lai et al., includes a pair of slides driven by a single motor, with a nonlinear crystal mounted to one of the slides. Keeping each slide fixed in one orthogonal direction allows the motion to be carried out, preferably in a circular or spiral pattern, while maintaining crystal axis orientation. This preserves the phase matching conditions necessary for optimal harmonic conversion.
Although the system of Lai et al., represents an improvement over other approaches involving manual and/or one-dimensional scanning or crystal translation techniques, it still suffers from certain shortcomings. In particular, while the heat load is effectively distributed over a larger interaction volume, namely the entire crystal surface, the techniques disclosed ignore the possibility that merely providing for continual repositioning of laser pulses over the crystal or optical element surface may not be sufficient to alleviate thermal damage concerns especially when repetition rates and power levels are increased to the multiple Watt levels currently of interest. For example, pulses may be inadvertently placed in an overly close temporal and spatial proximity do not take into account the potentially deleterious consequences of too much overlap between laser spots. Furthermore, variations in surface quality due to residual defects and inhomogeneities can result in non-uniform, unstable outputs. This may be completely unacceptable to semiconductor processing applications which require highly stable and uniform UV powers to guarantee repeatable effects in the processed materials.
There is a need for solid state laser systems, and their methods of use, that provide for prolonged life of optical elements exposed to high power laser radiation. There is a further need for solid state UV laser systems, and their methods of use, that provide for prolonged life of optical elements exposed to high power UV radiation, while maintaining output beam stability over long periods of time. There is yet another need for solid state, high repetition rate UV laser systems, and their methods of use, that provide for prolonged life of optical elements as average power levels increase, especially in high repetition rate solid state UV laser systems employing high intensity beams in various parts of the system.